1. Field of Invention
The field of the present invention relates in general to modems and more particularly to forward error correction in digital modems.
2. Description of the Related Art
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. With X-DSL significant increases in bandwidth have been made possible by utilizing frequencies higher than the voice band to deliver services such as: data, video, audio etc. Thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation. Currently there are over ten X-DSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there may be more than one line code, or modulation protocol, e.g. discrete multi-tone (DMT) and carrier less AM/PM (CAP). DMT modulation involves establishing a communication channel with a plurality of sub-channels each with a center frequency a.k.a. carrier tone. The sub-channels are frequency division multiplexed across the available bandwidth. Each sub-channel uses quadrature amplitude modulation (QAM) to modulate information.
Orthogonal Frequency Division Multiplexing (OFDM) is another modulation protocol which supports multiple sub-channels. OFDM is typically associated with digital communications over wireless communication mediums. In OFDM available bandwidth is subdivided into a number of discrete sub-channels that are overlapping and orthogonal to each other. Each channel has a corresponding frequency range. Data is transmitted in the form of symbols with a predefined duration. The data can be encoded in amplitude and/or phase, using encoding methods such as Binary Phase Shift Key (BPSK), Quadrature Phase Shift Key (QPSK), m-point Quadrature Amplitude Modulation (m-QAM).
Any digital communication experiences signal interference, and communication protocols which support multiple sub-channels such as DMT and OFDM are no exception. Interference can effect both the amplitude and the phase of the sub-channels. Such noise can arise across the time and/or frequency domains. At the receiver the data has to be separated from the noise. One popular technique for achieving the separation of data from the noise in a received signal is known as forward error correction (FEC). FEC introduces additional redundant bits into communications between modems and additional processing overhead to handle the transmission and reception of a stream of digital information. The redundant bits are added at the transmitter by application of any of a number of FEC algorithms in a process known as encoding the data. At the receiver the same algorithm is performed to detect and remove errors in the transmitted data in a process known as decoding the signal. The primary benefit of FEC is that re-transmission is not required.
With each improvement in bandwidth of multiple sub-channel communication systems there is a corresponding increase in noise, with the potential to reduce signal integrity to unacceptable levels. What is needed is a method and apparatus for increasing signal integrity in digital communication systems which support multiple sub-channels.